It is known to fabricate, using a unitary III-V semiconductor substrate, an integrated semiconductor device having a laser and a waveguide EA modulator. The modulator is formed using either a ridge waveguide or a buried mesa waveguide. The optical radiation output from the laser, for example visible or infra-red radiation, can then be coupled optically into the EA modulator, which is then used to impart a high frequency modulation on the optical radiation generated by the laser. Fabricating a laser and waveguide device on the same substrate gives significant advantages in terms of ensuring alignment between the laser and waveguide components of the device. The components can then use the same epitaxially grown current confinement layers, which helps to simplify the manufacturing process.
In the field of transmitter devices for fibre-optic communication, operation is required at optical wavelengths ranging from 1.3 to 1.6 μm. Such opto-electronic transmitter devices are therefore usually fabricated from a wafer grown from an n-InP substrate on which are grown a number of layers, including an undoped InGaAsP active layer, which is typically either a bulk semiconductor or a multiple quantum well or dot structure sandwiched between an upper p-InP cladding layer and a lower n-InP buffer layer. A mask is applied to the upper cladding layer, and the surrounding layers are etched to leave a mesa structure. Buried heterostructure light emitting devices commonly have current confinement regions defined by areas of high resistivity to limit current flow. Such regions are grown to cover the sides of the mesa and so channel electric current to an optically active layer within the mesa structure.
A mask defining the mesa is then removed, and further layers are grown up to a p+-InGaAs ternary cap layer. The ternary cap layer has a relatively low resistance and narrow bandgap facilitating electrical contact, and so serves as a contact layer to which electrical contacts may be made.
In devices using InGaAsP/InP materials, current confinement regions have often been employed based on a reverse-biased p-n or n-p diode structure. Such structures provide high resistance to current flow, and low leakage currents. These devices can also be directly modulated, and are widely used in fibre optic communication systems across a range of operating temperatures and at frequencies up to about 2.5 GHz.
In recent years there has been an increasing demand for fibre optic communication links having a bandwidth in excess of 2.5 GHz, for example up to at least 10 GHz. EA modulators can be used to achieve higher operating frequencies, but further limitations to operating frequency arise when an EA modulator is formed with the laser on the same substrate using the same current confinement structure. At operating frequencies above 2.5 GHz the performance of EA devices becomes limited by the capacitance of some of the conventional current blocking structures used by lasers. A lower capacitance structure that permits the EA modulator to operate at a high frequency may result in an additional current drawn by the laser, resulting in higher operating temperature. A laser can be stabilised against wavelength changes owing to temperature changes, for example by use of a distributed feedback grating, but temperature changes will adversely affect the performance of the EA modulator. It is possible to limit such temperature changes by use of a thermo-electric cooler, but this adds to the complexity and cost of the device.
It is an object of the present invention to provide a semiconductor device that addresses these issues.